PROJECT SUMMARY/ABSTRACT Protein modification has a variety of use cases, from cellular trafficking, targeted drug delivery, to imaging biomarkers. For a medicinal chemist, a single small molecule scaffold can set the stage for a nearly limitless number of controlled modifications to obtain an equally boundless set of well-defined derivatives for relevance in the context of pharmaceuticals and related industries. However, while the chemical biologist can imagine an infinite set of derivatives from a single protein scaffold, the tools available by which they may construct these bioconjugates is severely bounded therefore limiting their ability to test biological hypotheses. Moreover, the technology to enable large biomolecule coupling with stable linkages in stoichiometric quantities remains a formidable challenge and any such technology is projected to have a significant impact in the field. The preliminary results I have obtained not only enables robust, large protein-protein couplings at nanomolar concentrations, but more generally serves as a platform for rapid biomolecule diversification under biologically relevant conditions in the presence of diverse, unprotected functional groups. By using chemoselective palladium-thiol chemistry for chemoselective modification of low abundant cysteine residues, this proposal aims to expand the synthetic toolbox and chemical space available to chemical biologists. This is accomplished by the development of organometallic palladium reagents for the preparation stable, isolable, electrophilic organometallic palladium proteins for subsequent use in thiol couplings (Aims 1 and 2). To further demonstrate the utility of these organometallic palladium proteins, Aim 3 of this proposal will take advantage of this technology for the development of novel antibody-protein conjugates, an underexplored avenue of research due to the lack of technology to prepare these constructs. Thus, this proposal aims to expand the synthetic toolbox available for the chemical biologist to create a diversity of well-defined conjugates with the developed palladium reagents. Mindful of their practical use, reagents will be readily prepared, robust, and bench stable with protocols that can be easily implemented. With the goal of obtaining an academic job at a major university conducting interdisciplinary research, my training here at MIT will allow me to gain needed experience in chemical biology to complement my skills as a synthetic organic chemist. This will continue to entail hands-on research experience. In addition, I will have ample experience in writing, both in the context of proposal writing and manuscript preparation, mentoring, where I will continue to work with graduate and undergraduate students to hone this skill, and oral presentation skills by attending various conferences and guest lecturing courses. At MIT, I can continue to draw from the wealth of expertise in organometallic transformations from my colleagues in the Buchwald lab as well as that of chemical biology in the Pentelute lab making this the ideal environment for my post-doctoral training.